A conventional backlighting unit is described in U.S. Pat. No. 5,390,276. The principal element of this backlighting unit is a light guide, made of transparent material (for example Plexiglas), which possesses roughly the shape of a parallelepipedal plate. One of the end surfaces is used as a light entry surface, and the other end surfaces are treated, for example by metal-coating, in such a way that no light can emerge through them from the light guide. One of the two large covering surfaces is configured as a light exit surface, and small prisms are applied on the other large covering surface. The prisms all have the same cross-sectional shape and cross-sectional area, and all extend parallel to the light entry surface. The lamp is located in the immediate vicinity of the light entry surface. A cold-cathode fluorescent lamp, in the form of an elongated tube, is used as the lamp. The lamp is arranged in front of the light entry opening in such a way that as large a percentage as possible of the light radiation generated by the lamp is coupled into the waveguide. A reflector, which also conveys to the waveguide the light that is not irradiated directly into the waveguide, is additionally provided on the side of the lamp facing away from the waveguide.
As a result of the prisms arranged on the underside of the waveguide, the light present in the waveguide is coupled out in particularly efficient fashion by the fact that the light is reflected from the prisms applied on the one covering surface to the opposing covering surface, from which it can then emerge. It is important to consider in the case of this arrangement, however, that the coupling-out efficiency is the same over the entire waveguide. If the rod-shaped lamp is thus replaced, for example, by an L-shaped or
U-shaped tube, and if several end surfaces are thus used as light entry surfaces, the result is an inhomogeneous distribution of the light intensity in the waveguide. The homogeneous coupling-out efficiency thus results in inhomogeneous backlighting. On the other hand, however, the use of L-shaped or U-shaped tubes is desirable, since they are particularly efficient.